feat: add VAT tax rate, cost, and Letzshop feed settings

Product Model:
- Add tax_rate_percent (NOT NULL, default 17) for Luxembourg VAT
- Add cost_cents for profit calculation
- Add profit calculation properties: net_price, vat_amount, profit, margin
- Rename supplier_cost_cents to cost_cents

MarketplaceProduct Model:
- Add tax_rate_percent (NOT NULL, default 17)

Vendor Model (Letzshop feed settings):
- letzshop_default_tax_rate: Default VAT for new products (0, 3, 8, 14, 17)
- letzshop_boost_sort: Product sort priority (0.0-10.0)
- letzshop_delivery_method: nationwide, package_delivery, self_collect
- letzshop_preorder_days: Pre-order shipping delay

VAT Strategy:
- Store prices as gross (VAT-inclusive) for B2C
- Calculate net from gross when needed for profit
- Luxembourg VAT rates: 0%, 3%, 8%, 14%, 17%

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/vendor.py

@@ -57,6 +57,26 @@ class Vendor(Base, TimestampMixin):
     letzshop_csv_url_en = Column(String)  # URL for English CSV in Letzshop
     letzshop_csv_url_de = Column(String)  # URL for German CSV in Letzshop
 
+    # ========================================================================
+    # Letzshop Feed Settings (atalanda namespace)
+    # ========================================================================
+    # These are default values applied to all products in the Letzshop feed
+    # See https://letzshop.lu/en/dev#google_csv for documentation
+
+    # Default VAT rate for new products: 0 (exempt), 3 (super-reduced), 8 (reduced), 14 (intermediate), 17 (standard)
+    letzshop_default_tax_rate = Column(Integer, default=17, nullable=False)
+
+    # Product sort priority on Letzshop (0.0-10.0, higher = displayed first)
+    # Note: Having all products rated above 7 is not permitted by Letzshop
+    letzshop_boost_sort = Column(String(10), default="5.0")  # Stored as string for precision
+
+    # Delivery method: 'nationwide', 'package_delivery', 'self_collect' (comma-separated for multiple)
+    # 'nationwide' automatically includes package_delivery and self_collect
+    letzshop_delivery_method = Column(String(100), default="package_delivery")
+
+    # Pre-order days: number of days before item ships (default 1 day)
+    letzshop_preorder_days = Column(Integer, default=1)
+
     # Status (vendor-specific, can differ from company status)
     is_active = Column(
         Boolean, default=True